The inventors of the present invention have put a Faraday rotator made of a polycrystalline garnet (Tb3Ga5O12: TGG) ceramic having a Verdet constant substantially equivalent to that of the single crystal into practical use. The TGG, however, has a small Verdet constant in the visible light region, and thus downsizing of visible light lasers, which have been under aggressive development, is difficult. In addition, TGG is a comparatively expensive material, leading to a higher material cost.
To address these problems, single crystals of TAG (Tb3Al5O12), which has a higher Verdet constant than TGG in a wavelength region from visible wavelengths to around 1,000 nm and does not contain Ga as an expensive material, have been studied. TAG is, however, an incongruent melting compound, and thus it is difficult to directly produce, from a melted composition of a starting material, TAG single crystals corresponding to the garnet phase. On this account, a Czochralski process that enables production of large single crystals is difficult to be applied, and thus TAG single crystals are produced by a floating zone method (Patent Literature 1, Non-Patent Literature 1). By the floating zone method, large single crystals are difficult to be produced. Thus, there are various problems in mass production.
Patent Literature 2 discloses a method of producing a transparent rare earth aluminum garnet ceramic by controlling the average particle diameter and the molding density of a calcined powder to improve the transparency. However, due to the low transparency and the small thickness of 0.15 cm of the sample for characteristic evaluation, the practicability is poor.